In tear layer calculations for GP lenses, what constant is used in the tear layer formula tear layer power = (constant/diameter) - Ks?

• A. 337.5
• B. 340
• C. 360
• D. 320

Answer: (A)

In tear layer calculations for GP lenses, the tear layer power comes from a fixed conversion factor that translates the thin tear film between the lens and cornea into dioptric power. This constant sits in the numerator and the lens diameter (in millimeters) sits in the denominator, so the tear layer contribution decreases as the diameter grows. Subtracting Ks, the corneal curvature, accounts for the underlying anterior surface. That fixed constant is the standard value used in GP practice, chosen so the units come out correctly as diopters when diameter is in millimeters. Because this constant is fixed, the correct choice is the one that matches this standard conversion factor; the other numbers aren’t used in GP tear layer calculations and would misestimate the tear layer power.